Polymyxin is a generic term for a group of closely related antibiotic substances. Polymyxin B is the least toxic of these and is currently available for clinical use. Polymyxin B Sulfate is readily soluble in water. Aqueous solutions are quite stable at physiological pH. A 2% aqueous solution has a pH of about 5.7. The activity of Polymyxin B is restricted to gram-negative bacteria. A concentration of 0.1 to 0.25% in aqueous solution is said to be non-irritating and effective.
Neomycin is a complex of three compounds, Neomycins A, B & C. Commercial preparations are made with Neomycin B, which is a water soluble, thermostable substance. The sulfate complex is stable in the dry state, or in solution, at room temperature. It is a broad spectrum antibiotic, commonly marketed in dermatological and ophthalmic ointments, or as a sterile powder for dilution with saline solution for topical application or parenteral injection.
Several topical antibiotic formulations are known that employ Polymyxin B Sulfate in a suspension in combination with Neomycin Sulfate. Some such formulations include additional active ingredients, such a Lidocaine Hydrochloride and/or Zinc Bacitracin. These products are available in an oil base, such as in petrolatum, mineral oil, emulsifying wax, or a combination of those substances. Some examples of commercially sold formulations include the following:
Neosporin.TM. ointment: made of Polymyxin B Sulfate, Neomycin, Zinc Bacitracin, and Lidocaine, in emulsifying wax, mineral oil, purified water, and white petrolatum.
Campho-Phenique.TM. Triple Antibiotic Ointment Plus Pain Reliever: containing Zinc Bacitracin, Neomycin Sulfate, Polymyxin B Sulfate, and Lidocaine in a white petrolatum base.
Mycatracin.TM. Triple Antibiotic First Aid Ointment: Bacitracin, Neomycin Sulfate, Polymyxin B Sulfate in a microcrystalline wax, mineral oil and white petrolatum base. Micatrin.TM. Plus Pain Reliever also contains Lidocaine.
The base in these formulations, however, holds the actives in an oil phase, and does not allow quick, or sufficient, release to areas to be treated. Furthermore, application of these formulations in the oil phase creates the possibility of stained clothing.
Polymyxin B Sulfate has been used in combination with Neomycin Sulfate in specialized applications as well, including ophthalmic ointments (e.g., petrolatum based), otic sterile solutions, and ophthalmic sterile solutions. In a conventional ophthalmic ointment, antibiotics are suspended in petrolatum and mineral oil. Upon application of a dose to the eye, a portion is expelled, and spreading of the dose inhibited, because oily fluids are rejected. Furthermore, release of the active ingredient is delayed for the time required to exchange water from aqueous eye fluid. As a result, an inexact amount of the active ingredient reaches the site being treated. Solutions suffer similar deficiencies because they run out of the eye upon administration. Thus, with an ophthalmic ointment or solution, only a low, and imprecise amount of the antibiotic reaches the use point.